Flexographic printing elements are relief plates with image elements raised above open areas. Generally, the elements are somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based especially on their durability and the ease with which they can be made.
Flexography is commonly used for high-volume runs. Flexography can be advantageously employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. However, due to product competition, the market requirements on the printing quality of the images on the packaging can be very stringent.
A typical flexographic printing plate as delivered by its manufacturer, is a multi-layered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset printing processes on print quality. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
The support (or backing) layer lends support to the plate. The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. One widely used support layer is a flexible film of polyethylene terephthalate. In the case of CITR photopolymer sleeves, one preferred support layer is nickel.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If desired, more than one photocurable layer may be used.
Photocurable materials generally cross-link (i.e., cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. One commonly used source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art. Flexographic relief image printing elements are produced from photocurable printing blanks by imaging the photocurable printing blank to produce the desired relief image on the surface of the printing element, typically by selectively exposing the photocurable material to actinic radiation, which exposure acts to harden or crosslink the photocurable material in the irradiated areas.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is typically a slip film which has been modified to include a radiation opaque material. Portions of this “laser ablatable” layer are then ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. The benefit of using a laser to create the image is that the printer need not rely on the use of negatives and all their supporting equipment, and can rely instead on a scanned and stored image, which can be readily altered for different purposes, thus adding to the printer's convenience and flexibility.
The printing element is selectively exposed to actinic radiation in one of three related ways. In the first alternative (i.e., analog platemaking), a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative (i.e., digital platemaking), the photopolymer layer is coated with an actinic radiation (substantially) opaque layer that is sensitive to laser ablation and a laser is used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. In a third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer directly. Any of these alternative methods is acceptable, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush, the use of an air knife or heat plus a blotter (i.e., thermal development). Thermal development processes work by processing photopolymer printing plates using heat, and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images in order to go from plate to press more quickly, and processes have been developed whereby photopolymer printing plates are processed using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 7,122,295, 6,773,859, 5,279,697, 5,175,072 and 3,264,103, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for use in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
Once the heated printing element has been processed using heat, uncured photopolymer remaining can be melted or softened and removed. In most instances, the heated printing element is contacted with a material that will absorb or otherwise remove the softened or melted uncured photopolymer. This removal process is generally referred to as “blotting,” and is typically accomplished using a screen mesh or an absorbent fabric. Preferably, blotting is accomplished using rollers to bring the material and the heated printing plate element into intimate contact.
Upon completion of the development process (typically thermal or solvent), the printing plate element may be post-exposed to further actinic radiation in the same machine and/or subjected to detackification, and is then cooled and is ready for use.
The resulting surface, after development, has a relief pattern that reproduces the image to be printed and which typically includes both solid areas and patterned areas comprising a plurality of relief dots. After the relief image is developed, the relief image printing element may be mounted on a press and printing commenced.
While various improvements have been suggested to improve image fidelity and solid ink density (SID) in substantially planar relief image printing plates and to produce relief dots having desired geometric characteristics, the inventors of the present invention have determined that it would be desirable to investigate whether some of these same improvements would also demonstrate improved image fidelity, improved achievable SID, and dots having desired geometric characteristics, in cylindrical relief image printing elements.